1. Field of the Invention
The present invention relates to a control device for adjusting fuel injection of a diesel engine to compensate for fuel temperature variation.
2. Description of the Prior Art
A conventional diesel engine is generally not equipped with a temperature variation compensation means in its fuel injection pump for controlling fuel flow to match fuel temperature variation. Accordingly, the amount of fuel injection tends to vary with variation of fuel temperature.
In general, if the dynamic viscosity coefficient of the fuel decreases in accordance with a temperature rise, the amount of effective injection decreases due to an increase of fuel leakage in the injecting portion. The relative flow rate of the fuel thus tends to decrease by the variation in injection rate caused by variation of the volume elastic factor and by a decrease of mass per volume by expansion of the volume fuel. Generally a decrease of 9% of flow per 10.degree. C. variation in the fuel temperature results.
The temperature of the fuel varies with atmospheric temperature variation or by the amount of sun radiation incident on the fuel tank. Also since the cooling of the pump is affected by fuel circulating, the fuel temperature varies with the condition of the engine or the driving condition of the vehicle.
FIG. 1 shows variation of characteristic coefficients of an engine as a function of temperature variation. The graph is obtained by normalizing the factor at 30.degree. C. as factor 100 and shows output power and idling rotation. As shown in the figure, if the amount of fuel injection varies due to variation of the fuel temperature, the engine characteristics, for instance the output power and the idling speed of the engine, vary greatly at the upper and lower limits of accelerator operation. For example, if the various dimensions or design factors of the pump are decided by taking the fuel temperature at 30.degree. C. as the standard, the output power might increase by about 20% at a temperature of -20.degree. C. and generate more exhaust. On the contrary, if the fuel temperature rises to 70.degree. C., the output power decreases by about 20% and the idling rotation also decreases by about 10% and this causes an increase in engine vibration. Thus, operation of the conventional diesel engine is degraded by fuel temperature variation. As mentioned above in the FIG. 1, the graph shows in its ordinate the engine characteristic factors, assuming a factor of 100% at 30.degree. C. as 100% and the full line indicates output power and the dotted line indicates number of idling rotation. The abscissa is the fuel temperature plotted in .degree.C.